In CDMA communications, accurate detection of the data rate of the data in the channel is critical to proper transmission of data. Proper transmission and accurate reception of data in voice communications is in turn critical to acceptable sound quality. In CDMA standards IS-95 and J-STD-8 the data rate of the traffic channel can be one of four rates: full, half, quarter and eighth. The rate is selected by the vocoder depending on the voice activity factor. To reproduce the encoded and transmitted speech at the mobile station, the rate of the incoming frame must be determined precisely. Inaccurate rate detection results in annoying audible `pops` and `squeaks` and additionally in an increased frame error rate which is a critical measure of quality of a handset.
A previous rate detection method exists, I. Cohen and H. Lou "Multi-rate detection for IS-95 CDMA forward traffic channels" IEEE Globecom '95, November 1995 ("Cohen"). In Cohen, likelihood numbers are obtained by the absolute values of the sums of groups of the soft de-interleaved data, and then using thresholding methods to obtain the most likely rate of the incoming frame. Simulation of the Cohen method has been conducted and it has been determined that Cohen does not yield the desired performance.
A need exists in the art for high-performance rate detection methods and apparatus suitable for use with IS-95 and J-STD-8 CDMA traffic channels, which will precisely determine the rate of an incoming frame of data.